In recent years, there has been a trend to produce eyewear for use in active sports. Such eyewear takes the form of either unitary optical lenses that form face shields (i.e., visors) that are connected to helmets, or unitary or dual lenses that are more conventionally associated with specialized spectacles. For example, football and hockey helmets may have face shields attached to provide protection in a fairly wide field of view. These shields typically have a convex arcuate shape that curves at least in a horizontal plane and attaches to the helmet. Specialty sports glasses also have relatively large convex arcuate surfaces connected to frames that provide a shielding or protective function and thus have a fairly wide field of view.
The wide field of view is necessary because the sports practitioner must be able to direct his line of sight horizontally and vertically over wide angles in order to be able to perceive events that may be happening very quickly.
Many of the foregoing types of optical lenses are not totally enclosed by a frame. They are, at least on a portion of the edge of the lens, rimless. Typically, the rimless edge portions of eyeglass lenses are beveled in a V-shape or are rolled, i.e., they have a continuously curved edge. Unitary shield lenses on football and hockey helmets generally incorporate either a flat, rimless edge surface, which is approximately perpendicular to the front or back surface of the lens and has beveled edges, or a naturally convex arcuate shape, which is a side effect of the lens manufacturing process.
The traditional shapes of these exposed, rimless edges cause several detrimental visual effects including specular lens reflections, visual field interference (scotomatous ring), prismatic distortion, unwanted astigmatism, glare, multiple secondary images due to internal reflections, and ghost images. These effects are most pronounced when the wearer's line of sight is directed to a location that is near the rimless edge of the lens. For example, visual field loss at the exposed lower edge of a typical face shield lens having an average edge thickness is on the order of three-fourths of one degree (0.75°). Furthermore, if the edge is molded rather than cut to shape, there may be an additional 1.6° of high distortion in the exposed edge area. Thus, there may be a total visual field loss of approximately 2.4° at the exposed edge of a usable lens. While these amounts of “blind” areas sound small, they are significant because of the loss of functional visual space around much of the circumference of the lens and they can be detrimental to play. For example, a regulation American football thrown with a tight spiral subtends an angle of less than 0.75° at any distance greater than 16.7 yards; the same football viewed edgewise, as during a kickoff, subtends an angle of less than 0.75° at any distance greater than 25.1 yards.
In addition, disability glare and annoyance reflections, which are caused by internal reflection of a traditional exposed lens edge, may have detrimental effects on the wearer's ability to perceive objects near the edge of the lens and to maintain visual attention and focus. When combined with the visual field loss noted above, traditional lens designs have significant blind areas that prevent normal functional visual use in their exposed edge regions.